In recent years, the substitution of crosslinked rubbers by a thermoplastic elastomer advances from the viewpoints of recycle properties and energy conservation properties. Thermoplastic elastomers are constructed of a soft segment that is rich in rubber elasticity and a hard segment which will become a constraint phase of a molecular chain, and various thermoplastic elastomers using various combinations of a hard segment and a soft segment have been developed and put into the market. Above all, polyamide based or polyester based thermoplastic elastomers are broadly used in automobile parts, electric and electronic parts, industrial parts, and so on as a thermoplastic elastomer excellent in heat resistance and oil resistance. A hard segment of polyamide based or polyester based thermoplastic elastomers is constructed of a high-molecular compound having a high melting point, such as 6-nylon, 6,6-nylon, and poly(butylene terephthalate). However, there is a distribution in the hard segment chain length, resulting in problems such that the elasticity is lowered with an increase of the temperature, leading to a lowering of the softening temperature and that the temperature dependency of physical properties is large. Also, with the progress of lightening in the weight and thinning in the wall thickness, the creation of a thermoplastic elastomer having more excellent heat resistance is desired.
For the sake of solving the foregoing problems, a new type of thermoplastic elastomer containing as a hard segment an aromatic oligomer having a high crystallinity and a high melting point and having a uniform chain length are proposed. For example, a thermoplastic elastomer containing 4,4′-dihydroxy-p-quaterphenyl as a hard segment and an aliphatic polyester unit as a soft segment and having a low hardness and excellent heat resistance is proposed (for example, see Patent Document 1).
Also, a thermoplastic elastomer using an aromatic amide ester compound having a high crystallinity and a high melting point as a hard segment and a poly(oxyalkylene) unit as a soft segment is proposed. For example, a thermoplastic elastomer obtained by the melt polycondensation process of an aromatic amide ester compound composed of an aromatic dicarboxylic acid compound and an aromatic diamine compound and a poly(oxyalkylene glycol) (for example, see Patent Document 2); and a thermoplastic elastomer obtained by melt polycondensation of an aromatic amide ester compound composed of an aromatic dicarboxylic acid compound and an aromatic aminocarboxylic acid compound and a poly(oxyalkylene glycol) (for example, see Patent Document 3) are proposed.
Further, a thermoplastic elastomer obtained by the melt polycondensation process of an aromatic amide ester compound and a polyolefin polyol such as a hydroxyl-terminated ethylene/butylene copolymer (for example, see Patent Document 4); a thermoplastic elastomer obtained by polycondensation reaction of a terminal active aromatic amide ester compound and a polyester based polyol or a poly(organosiloxane) based polyol (for example, see Patent Document 5); and process of producing the same are proposed.
[Patent Document 1] JP-A-02-276817
[Patent Document 2] JP-A-06-207006
[Patent Document 3] JP-A-06-207007
[Patent Document 4] JP-A-08-253569
[Patent Document 5] JP-A-08-134210
(The term “JP-A” as used herein means an “unexamined published Japanese patent application”.)
However, according to the method proposed in Patent Document 1, since the reaction temperature is very high, the aliphatic polyester as the soft segment is liable to cause heat decomposition or depolymerization. As a result, not only the production of a high-molecular product is difficult, but also the chain length of the soft segment becomes non-uniform, possibly resulting in deterioration of mechanical characteristics.
Though the thermoplastic elastomers obtained by the methods proposed in Patent Documents 2, 3 and 4 had a high melting point and a high softening temperature, they were low in strength and insufficient in long-term heat resistance.
In the thermoplastic elastomer obtained by the method proposed in Patent Document 5, though it is admitted that the long-term heat resistance is improved, the problem that the strength is low is not solved yet. Also, expensive aminobenzoic acid is used in the synthesis of the terminal active aromatic amide compound, and complicated synthesis steps are required, resulting in a problem from the economical standpoint.